It is necessary for some vehicles to operate both on land and in water. These vehicles are generally tracked vehicles having ground laying tracks for land propulsion and screw-drives for water propulsion. The vehicles to date have utilized two separate drive systems, one for land and the other for water, which are driven by the same engine. This requires a separate land transmission including a torque converter and a separate device for dynamically engaging the engine to the sea-drive for providing a fixed ratio torque path.
The sea-drive requires a controlled engagement device, such as a fluid coupling, to permit smooth acceleration during engaging the high inertia drive system. These systems generally have some difficulty in converting from land to sea operation or vice versa. The speed of the engine is the one general road block to the interchange from land to sea-drive and vice versa. For example, if the engine is operating to drive the vehicle in water at approximately 25% of its maximum speed and the land transmission is engaged, the land transmission would also be driven at the same relative speed. Since there is not fixed relationship between the water speed and the land speed, or the speed over the sea bed, this would provide a very rough transition in the drive system from the sea-drive to the land-drive.